Electrochemical oxygen meter

ABSTRACT

An electrochemical device for measuring the concentration of oxygen in liquid alkali metal is described. Such device includes an elongated probe tube which is designed to be inserted into the liquid alkali metal whose oxygen content is of interest. A cup of a solid electrolyte material is bonded to the tube adjacent its lower end so that the outside surface of its bottom wall is in intimate contact with the liquid metal. The cup contains a mixture comprising a known concentration of one of the free metals gallium, indium and tin, and an oxide of such metal. This mixture is liquid at the temperature of operation of the device and is in intimate contact with the inner side of the bottom wall of the cup to provide a reference electrode. A high impedance volt meter is connected between the reference electrode provided by the mixture and the liquid alkali metal in order to provide a reading indicative of the EMF generated by ionic conduction of oxygen ions through the electrolyte and, hence, of the oxygen concentration in the alkali metal.

REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. patent application Ser. No.616,940, filed Sept. 26, 1975, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an electrochemical device for measuringthe concentration of oxygen in a liquid alkali metal and, moreparticularly, to such a device which has a longer life and is morereliable and accurate than prior devices of this nature especiallydesigned to measure the concentration of oxygen in molten sodium.

It is desirable for various purposes to be able to measure the oxygencontent of molten alkali metals used in industrial processes andcommercial equipment. For example, it is quite important to be able todetect the presence of oxygen in the liquid sodium heat transfer loopsof liquid metal fast breeder reactors. The presence of oxygen in theliquid sodium coolant of the primary coolant loop in such a reactor,i.e., the loop which passes through the reactor core, has to beminimized to prevent corrosion and consequent mass transport from thereactor core of radioactive corrosion products. A reliable oxygenmonitoring device is also needed for the secondary loop of such areactor liquid sodium coolant system in order to provide prompt andquantitative detection of steam or water leaks into the sodium.

Oxygen monitoring devices which rely on galvanic principles and ionicconduction have been designed to measure oxygen concentrations in moltenmetals. Basically, such devices provide an indication of the oxygencontent by measuring the electromotive force generated between areference electrode and a molten metal by the conduction of oxygen ionstherebetween through a solid electrolyte. The devices described in U.S.Pat. Nos. 3,776,831; 3,864,231; and 3,864,232 are representative of suchdevices. Presently available electrochemical oxygen monitoring devices,however, suffer from several deficiencies which make them less thanoptimum for use in measuring the oxygen content in liquid alkali metals,especially if the alkali metal is, for example, liquid sodium being usedas a fission reactor coolant.

One of the primary problems with most presently available devices isthat they are not as accurate as desired. That is, most of such devicesuse air or some other gas as a reference electrode, and in order toprovide a sufficiently fast response time the device must be operated ata relatively high temperature, e.g., 800° C. The difficulty withoperation of such a device with a gas reference electrode at such a hightemperature is that electronic conduction through the electrolytebecomes sufficiently high to interfere with the accurate measurement ofionic conduction through the solid electrolyte. Moreover, hightemperature operation substantially increases corrosive action of thealkali metal on the solid electrolyte, thereby reducing the effectivelife of the device. While it may appear that such problems could becircumvented by operating at a lower temperature, for example, attemperatures around 550° C., such devices generally become irreversiblewith consequent potential drift during operation.

Also most presently available devices will not provide accurate readingswhen initially immersed in a liquid alkali metal having a concentrationof oxygen in the range of parts per million. The electrolyte materialused in such devices is generally comprised of a stoichiometric ceramiccomposition that has oxygen atoms removed when initially immersed in thealkali metal until an oxygen-depleted composition in equilibrium withthe alkali metal is achieved. This removal of oxygen atoms from theelectrolyte interferes with the accuracy of the operation of the deviceuntil an electrolyte with an oxygen depleted composition is achieved.Generally the kinetics of such removal of oxygen atoms from theelectrolyte of the immersed device is very slow and a period of two orthree months is required until the electrolyte material is inequilibrium with the alkali metal.

SUMMARY OF THE INVENTION

The present invention provides an electrochemical device for measuringoxygen activity in an alkali metal which circumvents the above problems.As a particularly salient feature of the instant invention, instead ofthe reference electrode being air or some other gas, it is a uniformmixture of a known concentration of one of the metals gallium, indiumand tin, and an oxide of such metal, which mixture is liquid at thetemperature of operation of the device and is in intimate contact withthe solid electrolyte. Such a mixture has been found to be especiallyuseful as the material for such a reference electrode because of itsthermodynamic stability and lower melting temperature consistent with adesired lower temperature of operation of such a device. In order toeliminate the possibility of side reactions, the metal element selectedfor the mixture should also be the metal of the oxide.

The use of such a mixture as a reference electrode enables the oxygenmonitoring device of the invention to operate both reversibly andaccurately at temperatures within the range of between about 550° C. and650° C. In this connection, the use of such a mixture as a referenceelectrode also eliminates electronic conduction interference with thedesired measurement of ionic conduction of oxygen ions through the solidelectrolyte.

Another salient feature of the device of the instant invention is asolid electrolyte which has been found to be compatible and stable(resistant to corrosive attack) at temperatures as high as 800° C. withboth the preferred reference electrode mixtures and the alkali liquidmetals, such as sodium, whose oxygen content typically is of interest.Basically, the material of the solid electrolyte consists of high puritythoria doped with yttria, which material is sintered and fired at a hightemperature to obtain an actual density of 98% to 99% of its theoreticaldensity.

In a particularly preferred embodiment of the device of this invention,a solid electrolyte is employed that has improved properties. Thematerial of the solid electrolyte consists of high purity thoria dopedwith yttria, which material is sintered and fired for about two hours ata temperature of about 1650° C. and preferably at a temperature in therange of 1650° C. to 1700° C. in a reducing atmosphere of hydrogencontaining about 1 to 2 percent by volume water vapor. This reducingheat treatment depletes the composition of the electrolyte of sufficientoxygen atoms to yield a stoichiometry that does not have removal ofoxygen atoms when the electrolyte is initially immersed in an alkalineliquid metal having an oxygen content of about 1 to about 10 parts permillion (ppm).

The meter of this invention is also mechanically designed to obviatethose temperature variations in the electrolyte which cause many priordevices of this nature to be unreliable and shortlived. The mechanicaldesign of the device further simplifies the manner in which such deviceis mountable to a vessel containing the high temperature liquid alkalimetal whose oxygen content is to be monitored.

The invention includes other features and advantages which will bedescribed or will become apparent from the following more detaileddescription of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING

With reference to the accompanying single sheet of drawing, FIG. 1 is asomewhat schematic and partial cross-sectional view of a preferredembodiment of the invention mounted within a molten metal flow pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the drawing, a preferred embodiment of theelectrochemical oxygen monitor of the invention is generally referred toby the reference number 11. Such device is shown extending through thewall 12 of piping containing the flow of a liquid alkali metal such asmolten sodium 13. In this connection, the device 11 includes anelongated tube 14 of a non-corrosive metal which supports anelectrochemical cell 16 within the sodium flow. The upper end of theprobe tube 14, i.e., that end not in contact with the sodium, isprovided with an outwardly projecting circumferential flange 17. Suchflange mates with a corresponding flange 18 on the wall 12circumscribing the piping aperture through which the probe tube 14extends. Flanges 17 and 18 are suitably secured together, such as by acircumferential weld 19 as shown. Also, a top seal plate 21 closes theupper end of the tube 14 and seals its interior from the ambientatmosphere. Preferably, the interior of tube 14 is evacuated to, forexample, 10⁻³ torr so that thermal expansion and contraction of a gastherewithin will not cause structural problems.

Electrochemical cell 16 is in the form of a cup 22 secured within thetube 14 adjacent its lower end so as to expose the exterior surface ofits bottom wall 23 to intimate contact with the sodium 13. The cup 22 issecured within the tube 14 by a circumferential rim 24 circumscribingthe lower end thereof and engaging the inner wall surface of the tube14.

As will be discussed in more detail hereinafter, the material of the cup22 and rim 24 is a yttria doped thoria (YDT), and such cup is preferablysecured to the tube 14 with a fluid and gas tight seal formed bybrazing. The tube 14 is preferably stainless steel or nickel in order towithstand the high temperature corrosive environment of liquid sodium,and a suitable brazing material between such a metal and the yttriadoped thoria cup 22 is an intimate mixture of 70% by weight gallium and30% by weight nickel. It is preferable that the cup 22 be fired at 1000°C. for one hour in a hydrogen atmosphere prior to the time the braze isformed. The outer surface of the circumferential rim 24 is then wettedwith pure gallium before the cup is inserted into the end of the tube 14and a braze paste of the gallium and nickel is applied thereto. Thebraze joint is formed by heating the joint so assembled to 1350° C. andmaintaining the same at such temperature for three to five minutes undera vacuum of at least 10⁻⁶ torr. For best results, the braze joint isheld at 800° C. for about one-half hour as it cools from the brazingtemperature.

The bottom wall 23 of the cup 22 provides the oxygen ion conductive,solid electrolyte wall of the electrochemical cell. That is, its bottomor exterior surface is, as previously mentioned, in intimate contactwith the molten sodium, whereas its opposite surface, the bottominterior surface of the cup, is in intimate contact with a referenceelectrode. The material of the cup is high purity thoria (ThO₂) dopedwith between about 71/2% and about 15% by weight of yttria (Y₂ O₃),preferably about 71/2 to 8% by weight. The preferred range of 71/2 to 8%by weight optimizes the electrical conductivity. Most desirably, the cupis sintered and fired at a high temperature in order to obtain an actualdensity which is at least about 98% of theoretical density. It has beenfound that such a high purity, dense yttria doped thoria electrolyteresists attack by molten sodium up to 800° C. Also, it has been foundthat such an electrolyte will provide essentially one hundred percentionic conduction for oxygen ions at temperatures in the 550° C. to 650°C. range.

A particularly preferred process for making the cup involves mixing theThO₂ and Y₂ O₃ to achieve a homogeneous dispersion and forming a solidcup from the mixture by hot pressing at about 1900° C. at 4000 psi, thusachieving a sintered structure of at least about 98 percent oftheoretical density. This process achieves a homogeneous dispersion (asdetermined by electron microprobe analysis) of Y₂ O₃ in ThO₂ for optimumperformance of the device. The solid cup is then machined to the shapeof the cup 22. The machined cup is next subjected to a heat treatment toachieve a ceramic of a preferred stoichiometry namely an oxygen-depletedceramic of about 85 to 921/2 weight percent thoria with the balancebeing yttria. This is believed to yield, on a mole percent basis, thefollowing structure: (Th₀.70 to 0.85, Y₀.30 to 0.15) O_(2-x) where x isgreater than 0.075, with x being greater than about 0.076 when Th is 85mole percent and being greater than about 0.151 when Th is 70 molepercent. This heat treatment comprises firing the machined cup for abouttwo hours at a temperature of about 1650° C. to about 1700° C. under ahydrogen atmosphere containing from about 1 to about 2 percent watervapor. The resulting ceramic is in equilibrium with liquid sodiumcontaining from about 1 to about 10 ppm oxygen when immersed in theliquid sodium.

This heat treatment depletes the indicated amount of oxygen atoms fromthe electrolyte yielding a stoichiometry that does not involve theremoval of oxygen atoms from the electrolyte when it is immersed in aliquid alkali metal. In theory this process is believed to yield astoichiometry for the electrolyte having a Gibb's energy (ΔG₀₂ /mole O₂)that is substantially equal to the Gibb's energy exhibited by the liquidalkali metal, particularly liquid sodium, containing from about 1 toabout 10 ppm oxygen.

As a particularly salient feature of the instant invention, it includesa reference electrode which also provides essentially one hundredpercent ionic conduction in the 550° C. to 650° C. range. Anotherfeature of this invention is an electrolyte that is substantiallyisothermal during operation while immersed in the liquid/alkali metaldue to the limited size of the cup. In its basic aspects, the referenceelectrode comprises a mixture of one of the metals gallium, indium andtin, and an oxide of such metal. While the mixture ratios must be knownto allow calculation of the base oxygen activity in the referenceelectrode, the percentage of free metal to metal oxide is not crucialfor operation of the reference electrode. There must be, however,sufficient free metal in the mixture for it to be in contact with thesolid electrolyte wall 23 for the conduction of oxygen ions.

While from the theoretical standpoint a combination in the mixture ofone of the above metals and any of its oxides will produce the desiredresults, there are certain metal/metal oxide mixtures which areespecially suitable, particularly when it is the oxygen content ofmolten sodium which is of interest. That is, mixtures of tin (Sn) andstannic oxide (SnO₂); gallium (Ga) and gallium sesquioxide (Ga₂ O₃); andindium (In) and indium sesquioxide (In₂ O₃) are preferred. Both the freemetal and the oxide of each of these mixtures is a relatively lowviscosity liquid at the temperature of operation of the device (e.g.,550° to 650° C.) and have been found not to support electronicconduction interference with ionic conduction of oxygen ions through theelectrolyte at such temperatures.

The operating principle of a galvanic cell including a liquid mixture ofone of the above metals and its oxide as the reference electrode can berepresented as follows: ##EQU1## The difference in oxygen activityacross the electrolyte causes the ionic transport of oxygen through theYDT (yttria doped thoria) with a potential produced between thereference and sodium sides of the cell. The value of the open circuitpotential is thus a direct measure of the activity of oxygen in thesodium.

For the regime in which ionic conductivity in the electrolytepredominates (^(t) ion>0.99), the EMF of the cell is mathematicallygiven by:

    EMF=RT/4F log.sub.e P.sub.O.sbsb.2 Na/P.sub.O.sbsb.2 ref

where

F=Faraday Constant

R=Gas Constant

T-Temperature, °K.

P_(O).sbsb.2 =Oxygen Partial Pressure

By fixing the oxygen activity of the reference electrode and closelymonitoring cell temperature, the oxygen activity in sodium may bedirectly measured.

To obtain a measurement of such electromotive force, a high impedance(e.g., 10 megohm) volt meter which will draw insufficient current toaffect the readings, is connected between the liquid sodium and thereference electrode to measure the EMF. That is, a high impedance voltmeter 26 has one of its terminals connected via a lead 27 to the wall 12of the liquid sodium piping, which piping will be at the same potentialas the liquid sodium therein. The other terminal of the volt meter isconnected to a lead 28 of a refractory metal, such as tungsten ormolybdenum, which passes through seal plate 21 and the interior of theprobe tube 14 via an electrical feedthrough insulator tube 29. Asillustrated, tube 29 passes through a lid 31 on the cup 22, and lead 28extends therethrough into contact with the liquid metal/metal oxidereference electrode 32 therein. The purpose of the lid 31 is to preventreference electrode vapors from escaping from the cup and forming ashort circuit between the reference electrode 32 and the tube 14. Inthis connection, it should be noted that the tube 14 will be at the sameelectrical potential as the sodium 13.

In operation, it is desirable in order to reduce thermal shock that thetube 14 and electrolyte cup 16 be slowly heated to the temperature ofthe molten metal within which they are to be submerged, prior to thetime of such submerging. Then with the partial pressure of oxygen in thereference electrode known, the oxygen concentration in the molten sodiumcan be accurately determined by measuring both the temperature of themolten sodium adjacent the device and the EMF generated by the cell. Theequation set forth above can then be used to calculate the partialpressure of oxygen in the molten sodium. It should be noted thatprecalculated tables can be provided for an operator, setting forth thepartial pressure of oxygen in the molten sodium, with the temperatureand EMF as variables.

While the invention has been described in connection with a preferredembodiment thereof, it will be appreciated by those skilled in the artthat various changes and modifications can be made without departingfrom its spirit and scope. It is, therefore, intended that the coverageafforded applicant by the following claims be interpreted to encompassall reasonable changes and modifications.

We claim:
 1. An electrochemical device for measuring the concentrationof oxygen in a liquid alkali metal comprising:(a) an elongated tube of anoncorrosive metal having brazed in its open end by a braze materialconsisting essentially of an intimate mixture of gallium and nickel (b)a container defining an enclosed limited volume having on one side ofthe container an oxygen ion conductive, solid electrolyte body wallhaving a pair of opposite side surfaces, a first one of which surfacesis substantially even with and fills the open end of said tube and isadapted for intimate contact with the liquid metal whose oxygenconcentration is to be measured; (c) a mixture in the enclosed limitedvolume comprising a known concentration of one of the metals selectedfrom the group consisting of gallium, indium and tin, and an oxide ofsaid metal, which mixture is liquid at the temperature of operation ofsaid device and is in intimate contact with the second one of saidelectrolyte body wall side surfaces provide a reference electrode; and(d) means for measuring any electromotive force generated between saidreference electrode and said liquid metal by the conduction of oxygenions through said solid electrolyte to provide a reading indicative ofsaid oxygen concentration.
 2. The electrochemical device of claim 1 formeasuring the concentration of oxygen in a liquid metal in which theconstituents of said reference electrode mixture are selected from thegroup consisting essentially of tin and stannic oxide, gallium andgallium sesquioxide, and indium and indium sesquioxide.
 3. Theelectrochemical device of claim 2 for measuring the concentration ofoxygen in a liquid metal in which said ion conductive, solid electrolytebody wall consists essentially of a uniform mixture of about 71/2 toabout 15 percent by weight yttria with the balance being thoria and saidelectrolyte body wall has an actual density of at least about 98 percentof theoretical density.
 4. The electrochemical device of claim 1 formeasuring the concentration of oxygen in a liquid metal in which saidion conductive, solid electrolyte body wall consists essentially ofthoria doped with yttria.
 5. The electrochemical device of claim 4 formeasuring the concentration of oxygen in a liquid metal in which saidion conductive, solid electrolyte body wall consists essentially ofthoria doped with between about 71/2 and 8 percent by weight of yttria.6. The electrochemical device of claim 5 for measuring the concentrationof oxygen in a liquid metal in which said electrolyte body wall consistsessentially of about 921/2 percent by weight thoria and about 71/2percent by weight yttria and said electrolyte body wall has an actualdensity of at least about 98 percent of theoretical density.
 7. Theelectrochemical device of claim 1 for measuring the concentration ofoxygen in a liquid metal wherein said oxygen ion conductive, solidelectrolyte body wall is part of a cup of the material of saidelectrolyte which contains said mixture providing said referenceelectrode, and said cup is supported within the elongated tube whichextends into said liquid metal whose concentration of oxygen is to bemeasured.
 8. The electrochemical device of claim 7 for measuring theconcentration of oxygen in a liquid metal wherein said cup is positionedin said tube with its bottom wall being the surface exposed to contactwith the liquid metal.
 9. An electrochemical device for measuring theconcentration of oxygen in a liquid alkali metal comprising:(a) anelongated tube of a noncorrosive metal having brazed in its open end bya braze material consisting essentially of an intimate mixture ofgallium and nickel (b) a container defining an enclosed limited volumehaving on one side of the container an oxygen ion conductive, solidelectrolyte body wall comprised of a composition of a homogeneousdispersion of yttria in thoria, having a pair of opposite side surfaces,a first one of which surfaces is substantially even with and fills theopen end of said tube and is adapted for intimate contact with theliquid metal whose oxygen concentration is to be measured, saidcomposition comprising said wall being depleted of sufficient oxygenatoms so that upon immersion in the liquid alkali metal there issubstantially no removal of oxygen atoms from said wall; (c) a mixturein the enclosed limited volume comprising a know concentration of one ofthe metals selected from the group consisting of gallium, indium andtin, and an oxide of said metal, which mixture is liquid at thetemperature of operation of said device and is in intimate contact withthe second one of said electrolyte body wall side surfaces to provide areference electrode; and (d) means for measuring any electromotive forcegenerated between said reference electrode and said liquid metal by theconduction of oxygen ions through said solid electrolyte to provide areading indicative of said oxygen concentration.
 10. The electrochemicaldevice of claim 9 for measuring the concentration of oxygen in a liquidmetal in which the constituents of said reference electrode mixture areselected from the group consisting essentially of tin and stannic oxide,gallium and gallium sesquioxide, and indium and indium sesquioxide. 11.The electrochemical device of claim 10 for measuring the concentrationof oxygen in a liquid metal in which said oxygen depleted ionconductive, solid electrolyte body wall consists essentially of auniform mixture of about 71/2 to about 15 percent by weight yttria withthe balance being thoria and said electrolyte body wall has an actualdensity of at least about 98 percent of theoretical density.
 12. Theelectrochemical device of claim 9 for measuring the concentration ofoxygen in a liquid metal in which said oxygen depleted ion conductive,solid electrolyte body wall consists essentially of thoria doped withbetween about 71/2 and 8 percent by weight of yttria.
 13. Theelectrochemical device of claim 12 for measuring the concentration ofoxygen in a liquid metal in which said oxygen depleted electrolyte bodywall consists essentially of about 921/2 percent by weight thoria andabout 71/2 percent by weight yttria and said electrolyte body wall hasan actual density of at least about 98 percent of theoretical density.14. The electrochemical device of claim 9 for measuring theconcentration of oxygen in a liquid metal in which said oxygen ionconductive, solid electrolyte body wall is part of a cup of the materialof said electrolyte which contains said mixture providing said referenceelectrode, and said cup is supported within the elongated tube whichextends into said liquid metal whose concentration of oxygen is to bemeasured.
 15. The electrochemical device of claim 14 for measuring theconcentration of oxygen in a liquid metal wherein said cup is positionedin said tube with its bottom wall being the surface exposed to contactwith the liquid metal.